Automatic exposure control system for photographic printer



June 4, 1957 R. H. CANADAY 2,794,366

AUTOMATIC sxposuas: CONTROL SYSTEM FOR PHOTOGRAPHIC PRINTER F1106. Feb. 29, 1956 Roscoeflg'aggafi EN United States Patent AUTOMATIC EXPOSURE CONTROL SYSTEM FOR PHOTOGRAPHIC PRINTER Roscoe H. Canaday, Rochester, N. Y., assigns:- to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 29, 1956, Serial No. 568,468

4 Claims. (Cl. 88-24) The present invention relates to an automatic exposure control system for a photographic printer using as the light receptor a phototube of the electron multiplier type, and particularly to such a control system where changes in sensitivity of the phototube due to drift are minimized.

The use of automatic exposure control systems on photographic printers, both for making bl'ack-and-white and color prints, is well known, i. e., U. S. Patent 2,521,- 954. In such known systems all, or a portion, of the light passing through the negative to be printed is directed onto ,a phototube whose output is compared with a preselected standard value and any difference between the two is used as the basis for altering the exposure so as to correct for the difference and thereby produce the best print from the particular negative being measured. The exposure may in some instances be varied by keeping the time constant and varying the intensity of the printing light and in other instances the time of exposure may be altered and the intensity of the printing light be kept constant. Such control systems are rendered automatic by feeding any detected diiference between the output of the phototube and a preselected value into a servo system, or the equivalent thereof, which will in turn adjust the printing light intensity and/or the exposure time, as the case may be, to correct for this difference in measured and selected values and thereby produce the best possible print from the negative.

Because of certain characteristics which they possess, including a high amplification factor, a high sensitivity, etc., phototubes of the electron multiplier type have been known to be desirable for use in these exposure control systems since with some of the denser negatives there is a relatively small amount of light passing through the negative for measurement in a short time. However, this type of phototube has been found to be generally unsatisfactory in this application because of changes in sensitivity due to drift which are conducive to non-uniformity in print exposures, especially noticeable in repeat exposures from the same negative. While this change in sensitivity due to drift can be overcome to some extent by the use of complicated circuit modifications, such changes have proved to be so expensive and complicated that the use of electron multiplier phototubes has been less extensive in applications of this type than should be the case in view of their recognized advantageous characteristics.

It has been found experimentally that the stability of electron multiplier type phototubes is quite acceptable after a. few hours run at a constant illumination level and with fixed potentials applied thereto, but that each time the phototube output is subsequently changed, either by changes in illumination level or by changes in applied potential or both, the phototube drifts to a new sensitivity level. It, therefore, follows that this is the reason why phototubes of the electron multiplier type have presented such a problem when applied to automatic exposure control systems of photographic printers. This is the case because in such devices, while the normal control function is such that the phototube output remains substantially constant, between prints the phototube illumination may be much different than normal due to the fact that the illumination is reduced to a low level and/or may be entirely cut off from the phototube for purposes of viewing the negative or during periods of idleness.

The primary object of the present invention is to provide an automatic exposure control system for photographic printers using a phototube of the electron multiplier type in which changes in phototube sensitivity due to drift are minimized.

Another object is to provide an automatic exposure control system of the type described which includes a closed loop bias light control. The bias light is a controlled additional source of illumination which illuminates the phototube and acts to maintain a constant phototube output during printer operation as well as during standby intervals when the printer is idle.

And a further object is to provide an automatic exposure control system of the type set forth adapted for use on a photographic printer in which the printing light is reduced in intensity and/ or cut off from the phototube for viewing purposes, said control system including means for automatically introducing said bias control means into control relation with said phototube when the printing light is cut off from said phototube for viewing and for cutting ofi said bias control means when the normal printing beam is directed onto said phototube.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its methods of operation, together with additional objects and advantages thereof, will best he understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which:

Fig. 1 is a schematic showing of one form of photographic printer combined with an automatic exposure control system constructed in accordance with a preferred embodiment of the present invention; and

Fig. 2 is a detail view showing how the bias light control circuit may be switched into and out of control relation with the phototube by movement of the viewing mirror into and out of the printing beam.

Briefly, the present invention relates to an automatic exposure control system for use on a photographic printer in which during printing a portion of the light transmitted by the negative is reflected onto a phototube of the electron multiplier type whose output signal is then compared with a preselected value. Any difference between the output signal of the phototube and the preselected value is then used to adjust the intensity of the printing light until the output signal of the phototube is equal to this preselected value, or, stating it another way, until the intensity of the printing beam transmitted by the negative to be printed is equal to a preselected value. Prior to printing a negative, it is common practice to view the same for judging purposes, alignment in the gate, etc. and during this time the intensity of the printing light must be reduced lest the operator be blinded and/or it might be completely out off from the phototube if a swinging mirror is shifted into the printer beam to throw the image of the negative onto a viewing screen for this purpose. It has been found that the sensitivity of a phototube of the electron multiplier type changes due to drift if the output of the tube is repeatedly changed. Such a change in output will result from the change in intensity of the printing beam between normal printing conditions and viewing conditions or may result if for any reason the dynode voltage supply of the phototube is altered to change the sensitivity level of the tube.

I in order to minimize these undesirable changes in phototube sensitivity and make the use of an electron multiplier type phototube practical, the present exposure control system includes a closed bias light control which will keep the output of the phototube constant regardless of whether its output tends to change because of a change in the illumination of the phototube or because of a change in its dynode supply voltage. This bias control is so arranged that it is automatically connected into control relation with the phototube when, and only when, the intensity of the printing beam is reduced for viewing purposes or is cut off from said phototube entirely.

In Fig. 1 I have shown my automatic exposure control system combined with a known type of photographic printer in which a negative to be printed is illuminated by an incandescent lamp 11 and an enlarged image of which is projected by lens 12 onto a sheet of printing material 13. So that the negative may be visually judged prior to printing, a mirror 14 pivoted at 15 is adapted to swing to the dotted line position and throw the image of the negative onto a viewing screen 16. During the time a print is being made, the mirror assumes the solid line vertical position where it is out of the printing beam. A beam splitter 17 directs a fraction of the printing light onto a phototube 18 whose output is compared with a preselected value and any diiference is used to bring the intensity of the printing beam back to a preselected value after which the exposure is made for a preselected time. If the photographic printer is to be used for making color prints, a suitable printing filter 19 may be inserted into the printing beam and a suitable monitor filter 2%) may be inserted into the beam reflected to the phototube.

Up to this point the disclosed printer is conventional and the invention resides in the automatic exposure control system which will now be described. With this exposure control system the photocell tube 18 is one of the electron multiplier type, i. e. the RCA 931-A tube, having an anode 21, a cathode 22 and a plurality of other electrodes, commonly called dynodes, connected by suitable lead lines to a tapped resistance 23. The dynode input voltage is shown as derived from a source of potential, here shown as a battery 24, which includes an adjustable tap 25 by means of which the input voltage to the phototube can be varied to alter the sensitivity level of the tube. Connected in the anode return line 26 is an anode load resistor R1 and a source of potential shown as a battery 27 which produces a preselected reference voltage En against which the output signal of the phototube is compared for balancing purposes. The battery 27 is shown grounded at an intermediate point in order to establish the reference voltage ER at a given level above ground. Since the anode return line is connected to a source of constant voltage, if the dynode voltage happens to be changed, the printing intensity is then held at another level for all negative densities within the range of the system.

Any diflerence voltage Ea. resulting from a comparison of the reference voltage ER and the output of the phototube is fed by line 28 to a preamplifier A1 for amplification and then over line 29 to a power amplifier, designated generally as dotted box A2, which serves to adjust the intensity of the printing light 11 to correct for negative density and thus hold the printing intensity at a constant value as determined by a reference voltage ER. This power amplifier A2 serves the same purpose as servo systems commonly used in known photographic printers, i. e. like that shown in above-noted U. S. Patent 2,521,954, and including a reversible motor which responds to an error signal Ea. from the phototube circuit to adjust the intensity of the printing light by driving a rheostat in the lamp circuit, an adjustable diaphragm or density wedge in the printing beam, etc.

In this instance the power amplifier A2 is an amplifier which controls the current in the lamp circuit in response to the output signal Ea from the phototube circuit. This amplifier includes a source of potential 3!) which has one terminal connected to the lamp filament by line 31. The other side of the lamp filament is connected by line 32 to the plate circuit 33 of a plurality of pentodes 34, only two being shown here for simplicity but connected by dotted lines to show the existence of more, which are connected in parallel relation. The number of tubes 34 used in this amplifier will depend upon the wattage of the printing lamp and upon the characteristics of the tubes being used. For example, if a thousand-watt printing lamp is used with a 500 volt power source 30 and conventional 807 tubes are used which are capable of handling 25 watts of power each, then forty (40) tubes 34 will be required. If, on the other hand, a SOO-watt printing lamp is used, then only twenty (20) tubes will be required. The cathodes of the tubes 34 are connected to the other side of the power source 30 so that the current flowing in the tubes, and hence in the lamp circuit, will be regulated by the error voltage Ea. which is fed directly to the control grids 35 of the tubes. The screen grid circuit 36 is connected back to an intermediate point on the power source 30, shown at +250 v. for the present example where a 1000-watt printing lamp is used, and the suppressor grid circuit 37 of the tube is connected to the other side of the power source 30 and the cathode circuit.

Between prints, while the operator is viewing the negative, it is desired to hold the printing lamp intensity fairly constant at a lowered value which is comfortable for visual inspection of the negative by the operator. At the same time it is necessary to illuminate the phototube so as to produce normal output current, independent of dynode input voltage, in order to minimize a change in the sensitivity of the phototube as mentioned above. This is accomplished in accordance with the present invention by forcing the power amplifier input voltage to a selected negative level Ev by throwing the singlepole, double-throw switch 40 to the dotted line position.

This connects the negative bias Ev directly to the grid control circuit of the power amplifier A2 over lines 41, 41' and 29, thus holding the printing lamp current at the desired viewing level.

In order to maintain the output current of the phototube 18 constant during this viewing operation when the printing light is cut off therefrom, I provide a bias light control which will now be described. The photo tube 18 is adapted to be illuminated by a bias light 42, here shown as a neon lamp, one terminal of which is connected to a +300 v. supply and the other terminal of which is connected to ground through an electron tube 43. The intensity of illumination of bias light 42 is adjusted by a control circuit enclosed in a dotted line box designated A3. As pointed out above, the output of phototube 18 is compared with a preselected voltage EP. and the difference E3, is amplified by the preamplifier A1. The output of A1 is applied to the control grid 45 of tube 43 over line 44- through an adder comprised of resistances R2 and R3 and a lag network comprised of resistance R4 and condenser C1. The adder is used to inject the normal bias level as set by a slider on resistance R5 and reduce the loop gain to a reasonable level. The lag network R4, C1 shapes the frequency characteristics of the open loop to insure stable operation over a sufficient range of operating conditions. A remote cutoff pentodetype tube is used for tube 43 in order to obtain a plate current versus grid voltage characteristic which is nearly logarithmic over a large range. The effect of this, since the neon bias lamp 42 is very nearly linear with its current, is to minimize what would otherwise be wide variations in loop gain. This relationship simplifies the stablization of the loop and ensures that the control performance will be acceptable over the required range of bias light levels. The voltage divider R6 and R7 supplies screen voltage to the screen grid 46 of tube 43 during the viewing operation. Also connected to the screen of tube 43 by line 47 is the second pole of double throw switch 40 so that when the switch is thrown to the solid line position, the negative voltage Ev is applied to the screen of the tube 43 and biases this tube to cutoff. Thus, the bias light 42 is extinguished during the printing operation when the output of the phototube is normal. When the switch 40 is in its other, dotted iine, position, this bias is cut ofi from the screen grid 46 of tube 43 and the tube is allowed to conduct under the control of its control grid 45 whereby the intensity of the bias light is adjusted in response to the difference voltage Ea so that the output of the phototube 18 will be held at a preselected value determined by the value of reference voltage ER.

Switch 40 may be connected to the mirror 14 in any suitable manner, indicated by broken lines 50, so that when the mirror is swung to its inoperative position (solid. line position) for printing, the negative bias Ev is applied to the screen of control tube 43 to cut off this tube and extinguish bias light 42. When the mirror is thrown into the printing beam for viewing purposes (dotted line position) and the printing light is cut ofi from the phototube 18, the switch 40 is thrown to its dotted line position wherein the negative bias Ev is removed from the control tube 43 and this tube is free to conduct under control of its control grid 45 whereby the intensity of the bias light 42 is adjusted so as to keep the output of phototube 18 at a preselected normal value.

While the mirror 14 can be moved between its two positions in any suitable manner and the connection between the same and switch 40 can be of many known types, in Fig. 2 I have shown one way in which the desired result can be obtained. Here I have shown the mirror 14 provided with a handle 60 by means of which it is moved between its two positions. The single-pole, double-throw switch 40 is shown as being of the wellknown snap action type which is normally thrown to the dotted line position shown in Fig. 1 and is moved to the other (full line) position when a pivoted arm 61 having a roller on the end thereof is depressed against the switch housing. As shown, this switch is mounted on the printer in such a position that the arm 61 is engaged and depressed toward the switch housing by the mirror 14 when it is moved to its inoperative, or printing, position.

While I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted to the precise details of construction shown and described but is intended to cover all modifications coming within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In an exposure control system for a photographic printer the combination with means for projecting the image of a transparency onto a printing plane in which a sheet of light-sensitive printing material is adapted to be located and including a variable intensity printing lamp for illuminating said transparency from the rear; means for adjusting the intensity of said lamp until the light transmitted by said transparency reaches a preselected value and including a photomultiplier tube adapted to be illuminated by light transmitted by said transparency and whose output as compared to a reference voltage controls the intensity of said lamp; means for selectively cutting the transmitted image ofi from said printing plane and phototube and shifting it to a viewing plane for visual judgment; of bias control means for maintaining the output of said phototube constant at a value substantially equal to said reference voltage during the time the printing light is cut off from said phototube to eliminate a fluctuation in phototube sensitivity due to drift; and means for automatically introducing said bias control means into control relation with said phototube when the printing light is cut off from said phototube for viewing and for cutting out said bias control means when the printing beam is directed onto said phototube and printing plane.

2. An exposure control system for a photographic printer according to claim 1, in which said bias means comprises a lamp adapted to illuminate said phototube, and said last-mentioned means comprises a switch for turning on said lamp when the printing beam is cut off from said printing plane and phototube and for turning off said lamp when the printing beam is allowed to reach the printing plane and the phototube.

3. In an exposure control system for a photographic printer the combination with means for projecting the image of a transparency onto a printing plane in which a sheet of light-sensitive printing material is adapted to be located and including a variable intensity printing lamp for illuminating said transparency from the rear; means for adjusting the intensity of said lamp until the light transmitted by said transparency reaches a preselected value and including a photomultiplier tube adapted to be illuminated by light transmitted by said transparency and whose output as compared to a reference voltage controls the intensity of said lamp; means for selectively cutting the transmitted image off from said printing plane and phototube and shifting it to a viewing plane for visual judgment; of bias means for maintaining the output of said phototube constant at a value substantially equal to said reference voltage during the time the printing beam is cut ofi from said phototube to eliminate a fluctuation in the phototube sensitivity due to drift and including a bias lamp of variable intensity arranged to illuminate said phototube, a source of potential for said bias lamp, a control circuit for controlling the input current from said source to said lamp for varying the intensity of the same and including a grid controlled vacuum tube controlling the current to said lamp; means for impressing the output of said phototube as compared to said reference voltage on the grid of said vacuum tube; and means for automatically biasing said vacuum tube to cut off and connecting said printing lamp in control relation with said phototube when the printing beam is allowed to strike said phototube and printing plane and for removing the cut off bias from the vacuum tube and adjusting the intensity of the printing lamp to a predetermined intensity when the printing beam is cut ofi from the phototube for viewing purposes.

4. An exposure control system for a photographic printer according to claim 1, in which said bias lamp is a neon lamp whose light output is very nearly linear with its current and said grid controlled vacuum tube is a remote cut off pentode type tube having a plate current versus grid voltage characteristic which is nearly logarithmic over a large range to minimize what would otherwise be wide variations in loop gain.

No references cited. 

